That's interesting. Have you tried clearing S-voice? Settings > App Manager > ALL Apps > Scroll down and click on S-Voice > Clear Data.

Apparently there is an update in Android 4.3 that corrects some recognition issues with S-Voice.

You might want to consider doing that or just replacing Google Now with S-Voice when using headphones

I recently went from a Note II to the S4. Before I used s-voice mainly for text messing. I would say something like 'text john smith', I would then say what I wanted to send, it would interpret it, send the text. If there was a reply it would read any reply, and then asking me if I wanted to reply. Awesome.

Now with the S4, I have the same contacts and it's having a hard time even bringing the contact up. No matter how I say the contact it just doesn't work. Every once in a while it'll give me a list of contacts that are matches, but it comes no where near how accurate it was with the Note II. I haven't updated my s4 to 4.3 yet.

Does anyone know what is going on here? Is there an update to s-voice that corrects this problem?